Not sure if I understand you correctly, but this is how crops work in vanilla (Alpha 19)
When you plant a crop seed, you can get 1-3 at harvest depending on Living off the Land perk level (1 at no perk, 2 at perk level 1, and 3 at perk level 3). When you harvest it, the plant goes back to seedling at no cost to player.
Yes it takes 5 plants to craft into one seed so this is how it goes (if you simply harvest and turn into seeds) at Perk level 0
Plant 1 seed - 1 plant
Harvest 5 times, convert into seed, plant this new seed - Now you got 2 plants
Harvest 5 times, get 10 crops, convert into 2 seeds, plant both seeds - Now you got 4 plants
Harvest 5 times, get 20 crops, conver into 4 seeds, plant all those seeds - Now you got 8 plants
So yes you can get to sustainability